This invention is directed to transmitter-receiver equipments and more particularly to techniques for time division multiplexing the transmitting and receiving functions.
In the prior art, military aircraft equipped with both communications jamming equipment and communications intelligence receiving equipment had to utilize one of these two types of equipment at a time. Typically, the jamming equipment has to be turned off for a sufficient amount of time to receive enemy radio frequency transmissions in order to determine which signals should be jammed. For voice signals the listening interval must be undesireably long, in the order of seconds to tens of seconds, to determine the nature of the message. This must be done because linguistic analysis cannot be performed any faster. Also, certain data link sync. intervals may be as long as a second or more. During such unacceptably long listening. intervals the enemy is able to achieve communications which may have many purposes including the particular purpose of defeating the mission of the aircraft.
One unacceptable technique that has been tried has been to receive while concurrently transmitting jamming signals. This has not worked due to the limited isolation between transmitting and receiving antennas on an aircraft.
Another technique in use is to deliberately feed back some of the transmitted signal to the receiver input but shifted in phase to attempt to cancel out energy coupled from the transmitting antenna to the receiving antenna. However, this technique has not resolved the problem satisfactorily.
Thus, there is a need in the communications jamming art for apparatus and techniques that effectively provide full time receiving as well as full time jamming of enemy signal transmission
This need in the prior art is satisfied by our invention in which we utilize sampling techniques to effectively listen to and analyze received enemy transmissions while at the same time jamming same. More specifically, we utilize one or more search receivers to locate and analyze transmission to determine which received signals should be Jammed. However, these receivers only sample received signals at or above the Nyquist rate of the target signal""s modulation. The samples are applied to sample-and-hold circuits which-supply the missing energy during the periods the receivers are turned off. The sampling intervals occur at a rate such that the original modulation may be reconstructed. With the receivers used, an 85% jam duty cycle is obtained. What is, the receivers are utilized 15% of the time, typically thirty microseconds out of a two hundred microsecond interval, and while the receivers are disabled the jamming transmitter is energized. As the transmitter is typically energized for one hundred seventy microseconds out of each two hundred microsecond period, the jamming is essentially and effectively continuous. The effectiveness of the jamming is not impaired at all and the operator may listen to received signals as long as required, something that cannot be done with prior art systems wherein the transmitter sometimes has to disable for many seconds when received voice signals are being analyzed. The two hundred microsecond sample period is sampling at a five kilohertz rate which allows good reconstruction of speech and most other waveforms of interest for communications jamming.